The present invention relates to a branching filter for achieving separation between a transmitting signal and a receiving signal, a branching filter module, and the like, which are mounted to different radio communication apparatus, each of which shares the use of a transmitting and receiving antenna, and to a radio communication apparatus using branching filter module and the like.
As an example illustrating the structure of a conventional branching filter, reference is made to Japanese Patent Application Laid-Open Publication No. 62-171327.
The branching filter is constructed in such a manner that different elements necessary for the formation of the branching filter, such as transmitting and receiving signal filters for effecting filtering processes on transmitting and receiving signals using desired pass-band characteristics, signal transmission lines (formed by patterning, for example), an amplifying device or element, an interstage filter, etc. are all provided on the same substrate (called support base). Thus, the conventional branching filter or branching filter module wherein all the necessary elements have been provided on the same substrate, has generally been widely used.
Also, there has been proposed a branching filter wherein a package is formed by multilayer alumina made of alumina used as a material and an elastic surface-wave filter having a piezoelectric substrate and cord-like electrodes provided on the piezoelectric substrate is provided within the package, a branching filter module, etc.
The prior art is accompanied by a problem that since a two-branch circuit (e.g., a two-branch strip line) is provided on a single-layer type dielectric substrate, it is difficult to stack other circuit on the two-branch circuit and a size reduction becomes disadvantageous. This is because a strip line having a stable line impedance cannot be formed unless a conductor plate is structurally provided on a two-branch circuit (corresponding to a strip conductor) with a dielectric layer interposed therebetween.
In the structure wherein the two-branch circuit is simply disposed on the single-layer type dielectric substrate, the two-branch circuit is not constructed in the form of a closed structure. It was therefore necessary to provide shields or the like to reduce external noise and stabilize the line impedance.
Now, the closed structure is equivalent to a structure shown in FIG. 11. Namely, it is necessary to stack a dielectric on a conductor substrate in a state in which a conductive strip conductor (e.g., a two-branch circuit) has been embedded in order to form a strip line having a stable line impedance. It is also necessary to stack a conductor plate on the dielectric. Such a structure will hereinafter be referred to as a "closed structure". It is of course unnecessary to make the area of the conductor plate shown in FIG. 11 identical to that of the conductor substrate. However, the strip conductor needs to have an area corresponding to an extent sufficient that the strip conductor is interposed between the conductor substrate and the conductor plate with the dielectric interposed therebetween.
Since it is necessary to provide the conductor plate in this manner, the degree of freedom of laying out other circuit elements provided on the conductor substrate is limited.
It is also necessary to take into consideration impedance matching when it is desired to use a filter. It is therefore necessary to provided a matching circuit. However, the simple provision of the matching circuit on the support base makes it more difficult to reduce a size of a branching filter.
These problems can be solved more or less in the case of a branching filter using the aforementioned multilayer alumina. However, a problem still remains that the transmission loss of the two-branch circuit increases due to the difficulty of fabricating a multilayer alumina substrate and restrictions on characteristics of materials.